Alright, we have to ask ourselves "How much of 100% is one question worth?"
To find this, we must first find out how much was taken off of 100% to get 85%. In terms of math, this can be put like this:
100% - X = 85%
Now, we solve for X.
100% - X = 85%
Add X to both sides.
100% = 85% + X
Subtract 85% from both sides.
100% - 85% = X
Subtract.
15% = X
This means that 15% was taken off of 100% to get 85%. In other words, 3 incorrect questions are equal to 15% of the final grade. In terms of math, we can put it like this:
3 = 15%
Now, let's go back to the first question we asked at the beginning.
"How much of 100% is one question worth?"
To find this, we need to make 3 become 1 in the equation 3 = 15%.
3 = 15%
Divide both sides by three.
3/3 = 15%/3
Divide.
1 = 5%
This means that 1 question is worth 5% of the grade. With this information, we can find out how many questions it takes to get 100% of the grade. To do this, we need to make the 5% become 100%.
1 = 5%
Multiply both sides by 20.
20(1) = 20(5%)
20 = 100%
This means that there are a total of 20 questions on the test.